Method for crystallization of α-L-aspartyl-L-phenylalanine methyl ester

ABSTRACT

The problems in crystallization of α-L-aspartyl-L-phenylalanine methyl ester, namely, problems in crystal slurry properties in solid-liquid separation, scaling at heat transfer surfaces, and the like, are solved by a method for crystallization of α-L-aspartyl-L-phenylalanine methyl ester which comprises cooling a solution of α-L-aspartyl-L-phenylalanine methyl ester by indirect heat exchange with a coolant while stirring, wherein the solution is cooled by circulating a coolant while continuously adding an aqueous solution of α-L-aspartyl-L-phenylalanine methyl ester dropwise to a crystallizing solution of α-L-aspartyl-L-phenylalanine methyl ester having a temperature difference of not greater than 20° C. from the coolant, thereby to keep a temperature difference of not greater than 20° C. between the coolant and the crystallizing solution.

This application is a Continuation of application Ser. No. 07/967,234,filed on Oct. 27, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the preparation ofα-L-aspartyl-L-phenylalanine methyl ester (hereafter abbreviated asα-APM). α-APM is a peptide sweetener which exhibits a sweetness about200 times that of sucrose.

2. Discussion of the Background

α-APM has been widely used as a dieting sweetener in recent yearsbecause of its high quality sweetness and low calorie content. It isthus expected that its demand over the world will exceed 10,000 tonsthrough 1995.

The following examples of methods for preparing α-APM on an industrialscale are known:

(1) A method for preparing α-APM which comprises binding anN-substituted aspartic anhydride to L-phenylalanine methyl ester in anorganic solvent, splitting the substituent off in a conventional manner(U.S. Pat. No. 3,786,039), contacting the formed α-APM includingimpurities with a hydrohalogenic acid to obtain α-APM hydrohalide andthen neutralizing the hydrohalide;

(2) A method which comprises converting α-L-aspartyl-L-phenylalanine ina solvent mixture of water, methanol and hydrochloric acid into thecorresponding methyl ester to form α-APM hydrochloride, and neutralizingthe hydrochloride to give α-APM (Japanese Patent Application Laid-OpenNo. 53-82752); and

(3) A method which comprises condensing an N-substituted aspartic acidwith phenylalanine methyl ester in the presence of enzyme and thenremoving the substituent (Japanese Patent Application Laid-Open No.55-135595).

In the methods (1) through (3) described above, α-APM is eventuallycrystallized using a solvent which dissolves α-APM, such as water at ahigh temperature or a hydrated lower alcohol. The crystals are isolatedand dehydrated using an apparatus for solid-liquid separation, such as acentrifugal machine, and then dried to obtain the final product.

Crystallization methods include cooling crystallization, neutralizationcrystallization and concentration crystallization. However, α-APMdecomposes to diketopiperazine (and other compounds) at hightemperatures, such as those often employed in concentrationcrystallization. As a result, cooling crystallization is preferred whenconsidering the temperature stability of α-APM.

Cooling crystallization is generally carried out using a stirringcrystallizer having a heat transfer surface for cooling, or acrystallizer equipped with a heat exchanger in an external circulationsystem. When cooling crystallization is performed using a crystallizeraccompanied by forced fluidization, such as stirring or externalcirculation, fine needles of α-APM are usually obtained. Such fineneedles have poor properties for solid-liquid separation, such as a poorfilterability or dewaterability. As a result, conventional methods (forexample, a method for crystallization which comprises cooling a hotaqueous solution of APM by indirect heat transfer with a coolant at alow temperature) requires a large filtering area for solid-liquidseparation.

In addition, such methods result in crystals precipitating in largeamounts upon crystallization. The precipitated crystals adhere to thecooling surface, thus markedly reducing the cooling efficiency.

In order to efficiently cool a crystallizing solution, scales (adheredcrystals) must be removed from cooling surface. Continuouscrystallization generally provides high productivity on an industrialscale, however, if scaling occurs, it is necessary to stop operationoften. Then productivity is unavoidably reduced. Although scaling doesnot cause a serious problem in an experiment for a short period of timeusing a glass vessel, such as those frequently used in a researchlaboratory, scaling often creates a serious problem over longer periodsof time using a metallic apparatus, such as those used for continuouscrystallization on an industrial scale.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a novelmethod for the preparation of α-L-aspartyl-L-phenylalanine methyl ester,which reduces the amount of scale deposited on heat transfer surfaces ofa crystallizer.

A further object of the present invention is to provide a novel methodfor the crystallization of α-APM which improves the solid-liquidseparation properties of a crystal slurry of α-APM.

A further object of the present invention is to provide a markedimprovement of methods for crystallization of α-APM from a solution ofα-APM.

These and other objects which will become apparent during the followingdiscussion of the preferred embodiments have been provided by a methodfor the crystallization of α-APM, comprising cooling and stirring acrystallizing solution of α-APM, wherein the cooling is conducted byindirect heat exchange with a coolant, and the crystallizing solution ofα-APM having a temperature not greater than 20° C. higher than thetemperature of the coolant; and circulating the coolant whilecontinuously feeding an aqueous solution of α-APM to the crystallizingsolution of α-APM.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawing, wherein:

FIG. 1 sketches the method and apparatus used in Example 1; and

FIG. 2 shows the difference between direct (FIG. 2(A)) and indirect(FIG. 2(B)) cooling means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The problems described above can be overcome in continuouscrystallization by maintaining the temperature difference between thecrystallization solution and the coolant to 20° C. or less. Where thetemperature difference is more than 20° C., scaling occurs to aremarkable extent, and the temperature of the crystallizing solutionsubsequently increases. Accordingly, the operation must be discontinuedmany times to remove scales. Where the temperature difference is 10° C.or below, scaling is minimized. Therefore, maintaining the temperaturedifference between the crystallization solution and the coolant to 10°C. or less is extremely preferred in operation.

On the other hand, the aqueous solution of α-APM fed into thecrystallizing solution (the "feed" solution) should have as high aconcentration as possible, to maximize the productivity (efficiency fora given volume in an apparatus) and to obtain crystals having excellentsolid-liquid separation properties. As described above, however, α-APMis converted into diketopiperazine at a high temperature.Diketopiperazine is harmless (no toxicity), but exhibits no sweetness.Therefore, it is preferred that the temperature of the feed solution begenerally in the range of 30° to 80° C. and the solution concentrationbe about a saturation concentration.

The solvent is not limited to water, but the use of solvent mixture of alower alcohol (e.g., a C₁ -C₄ alcohol) and water is advantageous sincethe solubility of α-APM increases at high temperatures (for example,above 40° C.).

The present invention is effective in a semi-batch system, but exhibitsimproved effects in continuous operation. Thus, preferably, cooling andstirring the crystallizing solution are conducted simultaneously. Alsopreferred is cooling by indirect means; that is, heat transfer betweenthe coolant medium and the crystallizing solution occurs through apartition, such as the wall of a crystallizer, a coil in thecrystallizing solution through which coolant passes, a jacketsurrounding part or all of the crystallizing solution or thecrystallizer which conducts the coolant medium, etc. In the presentmethod, particularly preferred indirect cooling means include equippinga crystallizer with a jacket, a cooling plate, a coil or an externalheat exchanger.

As shown in FIG. 2, indirect cooling means differ from direct coolingmeans. FIG. 2(A) shows an example of direct cooling, in which heattransfer between the coolant medium 10 and the crystallizing solution 11occurs directly, by mixing the cooling medium directly with thecrystallizing solution. On the other hand, FIG. 2(B) shows an example ofindirect cooling, using a container 12 equipped with a jacket 13. Thepartition 14 between container 12 and jacket 13 conducts heat betweenthe coolant medium 15 and the crystallizing solution 16 in the apparatusof FIG. 2(B).

It is also preferred that the crystallization be conducted in acrystallizing apparatus having metal surfaces. Thus, the crystallizingsolution of α-APM is contained in an apparatus having at least one metalheat exchange surface. A particularly preferred metal for thecrystallizing apparatus is stainless steel.

The properties of the crystal slurry for solid-liquid separation areexcellent. The cooling efficiency of crystallizer is good, becausescaling occurs at a cooling surface only with difficulty. Continuousoperation can be performed over a long period of time.

Other features of the present invention will be come apparent in thecourse of the following descriptions of exemplary embodiments which aregiven for illustration of the invention, and are not intended to belimiting thereof.

EXAMPLE 1

As shown in FIG. 1, an α-APM slurry was charged in a stainless steelcrystallizer 1 (for industrial use) having a volume of 4 m³, equippedwith a cooling coil and stirrer A coolant having a temperature of 0° C.was circulated in the cooling coil, thus setting the temperature of theslurry at 5° C. A 3.5% by weight aqueous solution of α-APM having atemperature of 60° C. was continuously fed into the crystallizercontaining the cold slurry at a rate of 0.5 m³ /hr. The slurry waswithdrawn from the crystallizer and transferred to slurry holding tank 2at the same rate (0.5 m³ /hr), to keep a constant volume of slurry inthe crystallizer. The initial slurry was gradually replaced bycrystallizing slurry. The stirrer was set at 30 rpm. Crystallization wascarried out for four consecutive days, during which tile temperature ofslurry was kept at 4° to 6° C.

The slurry in slurry holding tunk 2 was immediately charged tocentrifuge 3 having an inner diameter of 48 inches for solid-liquidseparation (the corresponding centrifugal effect is 600 G). The watercontent in the every resulting wet crystals of α-APM was in the range of39-41%. After completion of the crystallization, the slurry remaining incrystallizer 1 was withdrawn, and the cooling surface was examined. Noserious scaling was noted.

EXAMPLE 2

The same procedures as in Example 1 were carried out, except that thetemperature of the slurry was kept at 15° C. or below (a maximumtemperature difference of 15° C. between coolant and slurry), and thecontinuous crystallizing operation was performed for 3 days. The watercontent in tile every thus-obtained wet crystals of α-APM was in therange of 38-46%. Slight scaling was noted.

Comparative Example 1

The same procedures as in Example 1 were carried out, except that thetemperature of the slurry was set at 25° C. Scaling occurred to aremarkable extent, and the cooling efficiency was reduced to such anextent that the temperature of 25° C. could not be maintained. Inaddition, the slurry temperature gradually increased to finally reach30° C. on the second day. Thereafter, the solution temperature showed atendency to still increase, and the total heat transfer coefficient atthe cooling surface clearly decreased. The situation indicated that nocrystal recovery is expected, and the crystallization operation wasdiscontinued. The water content of the final obtained wet crystals ofα-APM was 52%.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for crystallization ofα-L-aspartyl-L-phenylalanine methyl ester, comprising forming a coldsaturated solution of α-APM, which is formed by indirect heat exchangewith a coolant, and the said solution of α-APM having a temperature notgreater than 20° C. higher than the temperature of the coolant; andstirring said solution while continuously feeding an aqueous feedsolution of α-APM to the cold saturated solution of α-APM.
 2. The methodof claim 1, wherein said temperature difference between said coolant andsaid cold saturated solution of α-L-aspartyl-L-phenylalanine methylester is not greater than 10° C.
 3. The method of claim 1, wherein saidforming and stirring are conducted simultaneously.
 4. The method ofclaim 1, wherein said cold saturated solution of α-APM is contained inan apparatus having a metal heat exchange surface.
 5. The method ofclaim 4, wherein said metal is stainless steel.
 6. The method of claim1, further comprising the step of withdrawing an amount of said coldsaturated solution corresponding to the amount of said aqueous solutionof α-APM fed into said cold saturated solution.